"captured antenna" only exists in Cisco litterature for its Aironet products, this is kind of a trade mark as no other vendor uses this concept, so the translation is up to Cisco.
The previous answer is a Cisco-published French reference that can be used as is, although it does not explain much more in French than in English, for me it should be antenne captive" as this antenna is built into the transceiver and cannot be removed.